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West African Monsoon And Rainfall Enhancement Studies - Mali

Projects

Overview

Objective

Target Region: Mali within the bounded region. (click map to enlarge)

Water stresses often occur in West Africa. Increasing demands for water require that the potential for enhancing the sources, storage, and recycling of freshwater be examined carefully. Also, destruction and loss of life due to extreme weather events (e.g. flash flooding, extreme winds, etc.), which has the potential to be amplified with population growth and changing demographics, can have a large impact on human lives of the region. There is a need to examine ways to reduce these impacts. Recently, Mali has been conducting cloud seeding operations in hopes of augmenting precipitation to off-set the growing demands for water resources.

To increase our understanding of the the potential for cloud seeding to enhance rainfall, Mali has implemented an operational cloud seeding program to conduct these studies and assessment. NCAR is helping to support the operations by focusing on the evaluation and assessment studies on the potential for rainfall enhancement through cloud seeding experiments, airborne sampling, radar analysis, and randomized seeding trials.

Furthermore, understanding of the West African Monsoon and associated precipitation physics is critical in assessing the potential for cloud seeding to enhance rainfall in this region of the world. In addition, there is evidence that human activities such as the emission of industrial air pollution can alter atmospheric processes on scales ranging from local precipitation patterns to global climate (NRC, 2003). Documentation of anthropogenic effects on the weather strengthens the physical basis for the viability of deliberate attempts to alter the weather. However, to understand these inadvertent impacts on weather and climate require a concerted research effort.

Main Goal: To assist Mali and field personnel in conducting a feasibility study to assess the potential for rainfall enhancement using cloud seeding techniques. The feasibility study is conducted during the Monsoon season, which generally occurs between June and October. The program includes the latest technologies that have been developed in this field to conduct an airborne measurement program. The collaborative work will entail all necessary aspects of the project, including cloud physics and will build on the experience obtained in programs in other parts of the world. The first three years (2006, 2007, 2008) focused on sampling of clouds and aerosols with aircraft and radar observations along with a preliminary randomized seeding program. The 2009 program (June-September) will continue to sample clouds and aerosol along with the continuation of an exploratory randomized seeding program.

RADAR

Radar locations in Mali

The use of radar in rainfall and storm structure studies has become an important tool over the past twenty years. Because meteorological radars provide a wealth of information about precipitating cloud systems, it has also become essential to employ state-of-the-art software systems to display and analyze the data. While networks of weather radars are common in many western countries, large parts of Africa and other developing countries are currently not covered by weather radars. Recently, several African countries have also started to acquire weather radars but in many cases lacked the infrastructure to maintain, calibrate the radars, and interpret and analyze the data collected from these radars. Additional measurement capabilities in Mali and adjacent (e.g., Burkina and Senegal) regions will fill an important gap in the observational area of the Sahel, This region spans the transition zone between the Sahara desert regions and the more wet tropical southern areas of West Africa. The additional observational capabilities in this region could help in understanding the interaction between the Saharan dust layer and thetropical airmasses that propagate into the Sahel region from the south and the evolution of MCS's and CS's in this transition region and associated changes in rainfall patterns in the region.

Aircraft Observations

Of additional interest is the influence of aerosols and cloud microphysics (size and concentration of water droplets and ice particles inside clouds) on buoyancy, convergence, intensification of convection, and potential for enhancement of the natural precipitation. Aircraft operations are being conducted to assess the feasibility of any future precipitation enhancement potential in Mali. No previous airborne aerosol and microphysical measurements have been conducted in Mali. The aerosol and microphysical measurements will determine the optimal seeding method that may have potential for enhancing precipitation in Mali. The potential for such manmade increases is strongly dependent on the natural microphysics and dynamics of the clouds that are being seeded. These factors can differ significantly from one geographical region to another, and even between seasons in the same region. In some instances, clouds may not be suitable for seeding, or the frequency of occurrence of suitable clouds may be too low to warrant the investment in a cloud seeding program. Both factors need to be evaluated in a climatological sense. It is therefore important to conduct preliminary studies on the microphysics and dynamics of the naturally forming clouds prior to commencing a larger experiment. It is also important to conduct hydrological studies relating rainfall with river flows and reservoir levels, and to determine hydrological regions where reservoir catchments are most efficient.

An essential part of the radar analysis will be to determine the number of storms occurring over Mali. This is important in order to understand (1) the number of storms that occur naturally in the various regions around Mali, (2) the length of time that might be necessary in order to perform a later randomized experiment that would quantitatively describe the potential rainfall increase from seeding, (3) to assess the operational aircraft needs in treating these storms in a timely manner, and (4) to conduct a very preliminary estimate the overall area rainfall increases that might be possible from seeding. Using TITAN, the typical lifetimes, sizes, and intensities of rain events will be determined, in order to compare the Arabian storm climatology to those observed in other parts of the world.

The dynamical organization of storms responsible for the bulk of the rainfall will also be documented. A significant number of rainstorms are likely to be convective in nature (isolated storms as well as embedded). Among the convective storms, it is important to determine whether they are organized in individual convective units, or whether they often occur in organized lines. If there are a lot of line storms, then it may be necessary to adopt a different method of objectively characterizing the convective unit from what has been used in other recent seeding projects.

The software includes the NCAR Thunderstorm Identification Tracking Analysis and Nowcasting (TITAN), and the Configurable Integrated Data Display (CIDD), and Radar Echo Classifier (REC) software systems. Both the TITAN and CIDD displays have been implemented and will be used for viewing real-time and archived data. Using the TITAN system, data from the radars will be collected in volume-scan mode. TITAN identifies each storm seen by the radar, tags it with a specific identifier, determines the storm properties (such as height, volume, area, centroid, intensity, rainfall, speed of motion), and tracks it over time. The TITAN radar histories for a whole season can be stored on a single computer disk for later analysis and evaluation, making it an extremely useful archive and research tool. Overall, the radars will monitor the characteristics of the rainstorms to understand the following aspects: (1) the large-scale organization of the storms, (2) their frequency of occurrence and spatial distribution in the study area, (3) the temporal history, sizes, intensities and rainfall of individual storms, (4) the kinematic storm structures, and (5) divergence profiles from radar VAD profiles and aircraft measurements. An essential part of the radar analysis will be to determine the frequency of storms occurring over the various areas.

RT-FDDA Description

The RT-FDDA system was developed to provide high-resolution short-term analyses/forecasts (0-12 h). However, recent advances in computing power have allowed for a much longer forecast cycle; up to 36 h at current operational sites given the present grid and model physics configuration. In contrast, the twice-daily MM5 runs were specifically designed to provide long term forecasts (24-48 h).

RT-FDDA employs a time-continuous assimilation of a variety of synoptic and asynoptic observation data including:

METAR observations (includes "Specials")

Ship/buoy observations

Local surface observations

WMO rawinsonde observations

NESDIS satellite-derived winds

ACARS aircraft observations

These data sets have time frequencies varying from 5 min to 3 h, and are assimilated into the RT-FDDA system at their particular valid time.

By comparison, the twice-daily MM5 forecasts are limited to incorporating those observation data available at the synoptic times. These data are only used to improve the first guess at the initial time of the forecast cycle. Therefore, the twice-daily MM5 forecasts have a strong dependence on errors in the first guess. However, because the RT-FDDA cycles execute over a long period of time , errors can accumulate in regions without much data, although we have not observed major problems in this regard.

RT-FDDA analyses/forecasts do not generally suffer from model 'spin up' issues. Thus at any time, the RT-FDDA forecasts contain realistic and detailed mesoscale atmospheric structures, including cloud and precipitation systems, and local thermally-forced circulations. It should be noted that RT-FDDA does not assimilate cloud/precipitation data. The diagnosed cloud and precipitation systems in the analysis cycles result from the vertical motion and humidity assimilated from the available data.

The twice-daily MM5 forecasts, by comparison, are initialized using a 'cold start' methodology. This means that they start with no cloud and precipitation systems, or local thermally-driven circulations. Therefore, a certain amount of model 'spin up' time is required for the atmosphere, as it is represented by the MM5, to begin responding to the mesoscale forcing resulting from variations in the local complex physiography.

In summary, the characteristics of the RT-FDDA system generally contribute to a superior analysis/forecast compared to the twice daily MM5 forecast system. However, the advantages of RT-FDDA over the MM5 tend to decrease as the length of the forecast increases. This is principally due to the fact that the lateral boundary conditions employed by the MM5 and RT-FDDA systems are quite similar, and tend to have a stronger influence as the forecast length increases.

Lastly, the RT-FDDA system is temporarily employing a simple surface energy physics package. However, the RT-FDDA development team is busily working toward coupling Oregon State University land surface model (OSU LSM) to system. The new system incorporates many recent research/test results by the NCAR RTFDDA developers. Some major improvements are listed as following:

Land Surface Model (LSM): with more detailed and accurate soil physics than previous SLAB soil model.

Increase of the vertical model level from 31 to 36 and keep the level-distribution density with height. In other words, the resolution is increased in all troposphere and with more improvement in PBL layer.

An improved obs Quality_Control (QC) scheme that could effectively QC every kind of observations measured at any location, height and time. Previously only those obs that are located closed model 1st-guess levels were QC-ed.

More strict QC constraints. Working together with 3), it makes the system high quality and reliability.

Special Thanks go to the following members of the 4DWX team for their assistance in putting this system together: Yubao Liu, Laurie Carson, Becky Ruttenburg

These data sets have time frequencies varying from 5 min to 3 h, and are assimilated into the RT-FDDA system at their particular valid time. This allows the model to be nudged closer to observations before the next forecast cycle commences. Please refer to our page describing RT-FDDA for more information.

When does RT-FDDA work best?

RT-FDDA works best when there are a large amount of observations available for assimilation.

How do I read the forecast graphics?

What does forecast cycle mean?

A forecast cycle is the time in UTC the model starts running again. There is usually analysis of the previous 6 hours of model output and assimilation of observations that have become available for this time. The model then begins its forecast. The forecast period can last for up to 36 hours. For this particular implementation of the system, the forecast cycles run every 6 hours at 00, 06, 12, and 18 UTC. If you see graphics up there with a forecast cycle time more than 12 hours old, consider the forecast to be somewhat stale.

What is a cold start?

A cold start is when the RT-FDDA system uses model grids other than it's own to start a forecast cycle. These grids are commonly Eta, GFS, Ruc, etc.

Where can I find out more about the parameterizations used for this forecast?

How can I learn more about RT-FDDA?

Sponsors

National Meteorological Services - Mali

The National Meteorological Services of Mali provides the country's national weather and forecasting products.

Collaborators

National Center for Atmospheric Research - Research Applications Laboratory | Hydrometeorology Applications Program - Precipiation Enhancement Group

NCAR is a non-profit research and development center based in Boulder, Colorado sponsored by the National Science Foundation. The precipitation enhancement group at NCAR/RAL will be providing scientific guidance and the evaluation of the precipitation enhancement feasibility study.

University of North Dakota – Department of Atmospheric Sciences

UND is a public university located in Grand Forks, North Dakota. The Atmospheric Sciences department focuses on many areas of fundamental and applied research including airborne measurements of clouds and aerosols along with research and radar meteorology and mesoscale modeling.

Weather Modification Incorporated

Weather Modification Incorporated (WMI) is a privately held company incorporated in 1961 that provides weather modification equipment and services, cloud microphysical research, air pollution monitoring and aircraft modification. WMI is based in Fargo, North Dakota and is the primary contractor for the precipitation enhancement feasibility study in Mali.